The Mediterranean has long been characterized as a place of many languages. The term lingua franca, which has entered the English language as a description of any common language, originates in fact from the hybrid language used by merchants and travellers in the early modern Mediterranean. This paper aims to investigate multilingualism in the Jewish communities of the medieval Mediterranean: which languages were spoken or written, how they were learnt, in what contexts they were used, and how they interacted. Drawing on a wide variety of literary and documentary sources, including Genizah fragments, materia medica lists, chronicles and travelogues, this presentation presents evidence for the diversity of languages which were studied and spoken by Mediterranean Jewish communities, beginning in childhood and progressing through adulthood. I examine the languages used in various types of communal and professional contexts, focusing on the different linguistic needs of professions like medicine and trade. Exploring how language use and ability was affected by international trade and interreligious contact, and shaped by a speaker's economic and cultural position, this is the first comprehensive survey of Jewish multilingualism in the Middle Ages.